1. Field of the Invention
The present invention relates generally to an electroless plating technology in which a material to be plated is immersed in a plating solution to apply nickel plating, and particularly to an apparatus and a method for regenerating an electroless plating solution, which has been used in a plating treatment, in order to reuse it.
2. Description of the Related Art
Heretofore, in an electroless plating method in which, for example, hypophosphite is used as a reducing agent for applying nickel plating, a material prepared by combining nickel sulfate (nickel source) with sodium hypophosphite (reducing agent) is usually applied. When electroless nickel plating is conducted by the use of the plating solution having such a composition as described above, phosphite produced as a result of oxidation of hypophosphite, sulfate of a reaction residue derived from nickel sulfate used for the nickel source, and further, a sodium salt or ammonium hydroxide being a reaction residue of sodium hydroxide or ammonium hydroxide used for pH adjustment are accumulated in the plating solution over time, resulting in decrease in plating rate, the appearance of abnormal precipitation, deterioration of film characteristic properties and the like.
Under the circumstances, the plating solution is renewed after a certain period of service, while the used plating solution is treated as industrial waste. In this respect, however, since 1995, dumping of waste at sea has been banned under the Convention on the Prevention of Marine Pollution by Dumping of Waste and other Matter, and punishment for dumping waste on land has become severe. Therefore, it is very important to cut down the waste amount of the plating solution used, or to develop a method for regenerating the plating solution used.
Concerning the removal of sodium phosphite or sodium sulfate accumulated in the course of electroless nickel plating operation, a variety of approaches has been made already; the fact is, however, that none of these approaches can reach industrial practical application. For instance, as a method for removing sodium phosphite or sodium sulfate, a method for separating unnecessary components by means of electrodialysis with the use of a diaphragm is known. According to the method, however, there are such disadvantages that effective components are removed from the plating solution together with the unnecessary components, and in addition, that the apparatus therefor becomes big and expensive.
In R. W. Anderson et al., Plating and Surface Finishing, March 1992, there is proposed a regeneration method in which nickel ions and sodium ions have been previously separated by the use of an ion exchange resin from a plating solution, a calcium or magnesium salt is added to the remaining solution to insolubilize the sulfates and phosphites thereby separating them, so that the nickel and sodium ions which have been allowed to adsorb to the ion exchange resin are fractionated/desorbed, and then, only the nickel ions are returned to the plating solution to recycle it. However, the method involves technical and economical problems, so that the method has not yet been applied for practical use.
Furthermore, many other methods such as a method in which a plating solution is cooled to crystallize sodium sulfate thereby separating the sodium sulfate, and then calcium sulfate and calcium hydroxide is added to the plating solution so as to be 5.5 to 7.0 pH, whereby only the phosphite is selectively separated and removed as a calcium salt (Jpn. Pat. Appln. KOKOKU Publication No. 36-3557); a method in which ammonium hydroxide has been previously added to a plating solution, then, a hydroxide such as calcium, barium, and strontium is added to the plating solution to selectively precipitate and separate only the phosphite from the plating solution; or a hydroxide of an alkaline earth metal is added, then, sulfuric acid is added to the plating solution so as to obtain 4 to 6 pH to precipitate and separate the phosphite, whereby the plating solution is regenerated (Japanese Patent No. 2769774) are proposed. However, any of these involves many problems in view of practical use.
Namely, a conventional technology for generating an electroless plating solution involves the problems as mentioned hereinafter.
A large amount of a sulfate existing in an electroless plating solution accompanies not only extreme difficulty in separation from the plating solution, but also increase in a solubility of calcium phosphite, so that it becomes a significant disability for separating the phosphite. Thus, the plating solution used in the present invention is based on the premise of containing no sulfate.
More specifically, a part of a plating solution is drawn, calcium hydroxide or calcium carbonate is added to the plating solution so drawn to fix it as calcium phosphite in an electroless plating method in which a material to be plated is immersed into a plating solution in which nickel hypophosphite is used as a nickel source and at least one or more members selected from nickel hypophosphite, hypophosphorous acid, and sodium hypophosphite is used together as a reducing agent, whereby nickel plating is conducted.
The present inventors have proposed in Jpn. Pat. Appln. KOKAI Publication No. 2001-192849 that an amount of a calcium salt to be added is adjusted such that a pH of a plating solution does not exceed 5.8, whereby only the phosphite which is a component unnecessary for plating is separated and removed from the system, so that the resulting plating solution is recycled without losing effective plating components.
Thereafter, there has been proposed such a method and an apparatus for liquid management in Jpn. Pat. Appln. KOKAI Publication No. 2002-241952 that a drawing rate of a part of a plating solution to be drawn from the plating solution and an amount of calcium hydroxide or calcium carbonate to be added for precipitating and separating the phosphite is adjusted in an amount corresponding to that of the phosphite which will be produced by the plating reaction, whereby a concentration of the phosphite in the plating solution is maintained at a value within a range (50 to 100 g/l) which has been previously set, and further none of the components not required for the plating reaction is applied.
However, since there is no contrivance for automation as to the drawn rate of the plating solution for separating the phosphite, it is required to calculate an amount of the plating solution to be drawn and an amount of the calcium hydroxide or calcium carbonate to be added in each case based on the plating throughput to be processed, and thus, there is a problem of poor multiusability.